Generally, the soldering of metals is effected by applying a soldering medium or flux on the place to be soldered, and then applying the solder with a soldering iron or the like. Alternatively, solder preforms have been used wherein a preform is heated to a molten state whereby it forms a good solder bond. Various preforms must be made for each individual situation. Consequently, the entire soldering process is very time consuming and expensive.
Various attempts have been made to produce printable (e.g. by screen or stencil techniques) solder compositions or pastes. The use of solder pastes which are composed of powdered solder alloy dispersed in a relatively small volume of a vehicle, is experiencing wider acceptance in industry primarily because pastes are more readily adapted to automated manufacturing procedures, than are the conventional manually operated solder solder bar, separate flux and iron or torch, or solder wire containing flux in a core within the wire and an iron or torch. The make-up of prior solder paste compositions has been influenced largely by prior practices, particularly in respect to fluxing or other surface preparing agents. These have comprised substantial portions of organic acids, inorganic acids and salts thereof, e.g., zinc chloride, ammonium chloride, and an additional alkali metal halide or ammonium halide other than chloride, either in dry form or mixed in a suitable solvent. However, the additional alkali metal halide or ammonium halide when added to the conventional zinc chloride, ammonium chloride flux has been found to either leave an undesirable corrosive halide residue on the metal surface or has resulted in a roughened surface on the finally joined members.
Japanese No. 64[1989]-40197 discloses a flux consisting of 1-20 weight % of tin fluoride and the balance of potassium fluoroaluminate complex. It is stated that use of greater than 20% of the tin fluoride results in the brazing properties being degraded, and use of less than 1% results in the effect being insignificant.
Japanese No. 62[1987]-16898 discloses a type of flux for soldering characterized in that it contains stannous fluoride as the activator for the soldering flux. The stannous fluoride is used in combination with well known binders such as rosin, rosin phenol, polyethylene glycol, etc. The fluxes are used in combination with solder baths.
U.S. patent application, Ser. No. 07/222,496 filed 7/21/88 and now U.S. Pat. No. 4,867,800 discloses in reflow soldering, a solder paste, containing both flux and solder metal in powder form, is applied only to the points where solder bonds are to be made. However, these pastes have been found to require the use of cleaning agents which still leave some ionic contamination after cleaning. Other cleaning agents such as chlorofluorocarbon (CFC) solvents remove all residual ionic contaminants after the soldering operation but have an undesirable environmental effect.
It has been found that the above disadvantages can be overcome and improved solder pastes having low ionic contamination after use of the paste in a soldering operation can be prepared by adding stannous fluoride to conventional solder pastes comprising a metal or metal alloy solder powder and a carrier or paste vehicle.